In general, there are two types of vertical cavity surface emitting lasers (VCSEL), bottom emitting and top emitting. Bottom emitting VCSELs have an emitting area on the back side of the substrate on which they are formed. This type of VCSEL is utilized because of the ease with which the electrical contacts are formed. The top can be covered with an electrical contact and the substrate can be a highly doped semiconductor which renders it conductive. However, in addition to the pure physical constraints (e.g. mounting problems) there are many other limitations as a result of the substrate being in the emitting path.
Prior art top emitting VCSELs are more difficult to construct and include some current limitations as a result of the construction. In top emitting VCSELs a mirror stack is formed on the substrate and an active and spacer layer is formed on the mirror stack. A second mirror stack is placed on the active and spacer layer and an electrical contact is placed on the second mirror stack so as to define an emitting opening therethrough. This configuration produces some current confinement problems and instabilities in the optical emission. However, the top emitting VCSEL has the advantages of; 1) the lasing wavelength is not constrained by the bandgap of the substrate material; and 2) the coupling of the light output to external equipment is not hindered by the substrate for closely packed array applications.